The purpose of this research will be to study the alterations in water and electrolyte balance in skeletal muscle of man during prolonged work in the heat. The primary objective of these investigations will be to assess the effects of dehydration on the distribution of water and electrolytes between the extracellular and intramuscular compartments. Additional observations will be made to relate these changes to the contents of extra-cellular fluids and the composition of sweat and urine. A muscle biopsy procedure will be utilized to obtain muscle samples before and at various levels of dehydration. Muscle samples will be assayed for water, sodium chloride, potassium, magnesium, glycogen, and lactate. Blood samples will be taken at rest and after dehydration to determine the changes in electrolytes, glucose, lactate and plasma volume. The following studies will be conducted in order to satisfy the objectives: (1) water and electrolyte content of working and inactive muscle tissue after dehydration; (2) effects of varied levels of dehydration; (3) water and electrolyte concentrations in selected muscle fibers; (4) effects of successive days of dehydration - rehydration; (5) effects of electrolyte replacement during repeated days of dehydration - rehydration.